Antimicrobial in Equine Medicine

What are pros of beta-lactams?

• safe in horses

• high efficacy

• low cost

what is the mechanism of action of beta-lactams?

interfere with bacterial cell wall production —> cell lysis

What do we use beta-lactams for?

• gram positive bacteria

• best choice for streptococci infection

• most anaerobic infections

what other antimicrobial can we use with beta-lactams for a better effect?

• aminoglycosides

• fluoroquinolones

what are cons of beta-lactams?

limited efficacy with abscesses (which are common with streptococci infection)

what are the routes of administration of beta-lactams in horses?

• Na and K penicillin - IV

• Procaine penicillin - IM

• oral absorption is too poor in horses

what are adverse effects of beta-lactams?

• type I immune reactions - anaphylaxis

• type II immune reactions - haemolytic anaemia, thrombocytopenia

what do we need to be aware of when administering procaine penicillin?

• horse tests positive in dope test for next 6 weeks (important for race horses)

• if bottle not shaken properly to mix - can administer pure procaine —> overdose

• if given IV instead of IM - CNS effects

what is the effect of beta-lactams on the bacteria?

bactericidal - kill the bacteria

what is the mechanism of action of cephalosporins?

interfere with bacterial cell wall production —> cell lysis

(same as beta-lactams)

why might cephalosporins be a better choice than beta-lactams?

same mechanism of action - however cephalosporins more resistant to bacterial defences

what are the main differences between the generations of cephalosporins?

the higher the generation (4th gen highest) the more effective they are against gram negative bacteria

what is the effect of cephalosporins on the bacteria?

bactericidal - kill the bacteria

what is the mechanism of action of aminoglycosides?

• penetrate bacteria

• bind to ribosomal subunit —> cause misreading of genetic code —> inhibit production of proteins

what would we use aminoglycosides for?

• gram negative bacteria

• especially effective for pseudomonas infection

what is the route of administration for aminoglycosides?

• poor oral absorption

• good via injection

what are adverse effects of aminoglycosides?

• nephrotoxicity

when would we avoid using aminoglycosides?

• dehydrated animals

• if other nephrotoxic drugs are being used

• animals with any sign of renal damage

what is the effect of aminoglycosides on the bacteria?

bactericidal - kill the bacteria

what is the mechanism of action of chloramphenicol?

• bind to ribosomal subunit

• inhibit protein synthesis

what is the effect of chloramphenicol on the bacteria?

bacteriostatic - don’t kill the cell, just damage it and inhibit growth

when would we not use chloramphenicol?

in immunosuppressed animals - as only inhibits growth of bacteria, doesn’t kill it (relies on immune system)

can chloramphenicol be used in food producing animals?

no - if giving to horses, make sure to sign them off the food chain

what are routes of administration of chloramphenicol?

• good oral absorption

• not given IV due to short half life

• not given IM as very painful

what are adverse effects of chloramphenicol?

• colitis

• aplastic anaemia in humans - so not given to food producing animals

what can we not administer chloramphenicol with?

• beta-lactams

• aminoglycosides

• fluoroquinolones

• macrolides

what are potentiated sulphonamides?

combination of sulphonamide and diaminopyrimidine

what is the mechanism of action of potentiated sulphonamides?

• inhibit folic acid pathway —> block nucleic acid synthesis

• bacteria cannot produce genetic material

what is the effect of potentiated sulphonamides on the bacteria?

• sulphonamide is bacteriostatic - don’t kill bacteria, just inhibit growth

• when combined with diaminopyrimidine (potentiated sulphonamide) —> bactericidal

what are cons of potentiated sulphonamides?

• ineffective in pus and necrotic tissue

• ineffective against most anaerobes

what would we use potentiated sulphonamides for?

• strep

• staph

• some gram negative - E. coli, Salmonella

how do we administer potentiated sulphonamides?

• good oral absorption

• rapid IV admin causes collapse

what are adverse effects of potentiated sulphonamides?

• anaemia, thrombocytopenia

• crystalluria

• if given IV with detomidine —> fatal dysrhythmias

What is the mechanism of action of tetracyclines?

• bind to ribosomal subunit —> inhibit protein synthesis

what is the effect of tetracyclines on the bacteria?

bacteriostatic

what would we use tetracyclines for?

• gram positive and negative

• some anaerobes

• chlamydia

• mycoplasma

• ehrlichia

• can be used to treat contracted tendons in foals

how are pros of tetracyclines?

very lipid soluble so can be used for abscesses

how do we administer tetracyclines?

• IV - tetracycline

  • rapid IV can lead to collapse

• oral - doxycycline

what are adverse effects of tetracyclines?

• fatal colitis

• discolouration of teeth

what is the mechanism of action of fluoroquinolones?

• inhibit bacterial DNA gyrase

• abnormal configuration of DNA

• autolysis

what is the effect of fluoroquinolones on the bacteria?

bactericidal

what are fluoroquinolones used for?

• most aerobic gram negative

• some aerobic gram positive

• mycoplasma

• chlamydia

• best choice for salmonella

what are the routes of administration for fluoroquinolones?

• good oral absorption

• can be injected aswell

what are pros of fluoroquinolones?

very lipid soluble - can be used for abscesses

what are adverse effects of fluoroquinolones?

cartilage lesions in foals

what antimicrobials should we not use fluoroquinolones with?

antimicrobials that inhibit bacterial protein synthesis

what is the mechanism of action of macrolides?

binds to ribosomal subunit —> inhibit protein synthesis

what is the effect of macrolides on the bacteria?

bacteriostatic

what is a con of macrolides?

resistance develops quickly

what age of horses would we never use macrolides in?

adult horses - causes colitis

what would we use macrolides for?

• treatment of Rhodococcus equi in foals

• can give in very small IV dose as a prokinetic (only time we can use in adult horses) - colitis is still a concern

what is the route of administration of macrolides?

• oral most common - via tablets

• can be injected

what are adverse effects of macrolides?

• colitis and diarrhoea in adult horses

• hyperthermia

what is the mechanism of action of rifampin?

• inhibit bacterial RNA polymerase

• decreases RNA synthesis

what are cons of rifampin?

rapid development of resistance - so we use in combination with other antimicrobials

what are pros of rifampin?

good penetration - can be used for abscesses

what do we use rifampin for?

• staph

• Rhodococcus equi

• mycobacteria

• some viruses and fungi

what is the best treatment of Rhodococcus equi?

rifampin + erythromycin

what is the route of administration of rifampin?

• can be given oral

• can be injected

what are adverse effects of rifampin?

stains everything red - urine, faeces, tears, saliva, etc.

what is the mechanism of action of metronidazole?

• anaerobic bacteria take it up and break into small free radicals

• causes DNA damage

what is metronidazole used for?

• anaerobic bacteria

• protozoa (but rare in horses)

what is the route of administration of metronidazole?

• can be given oral - but not very palatable, and makes you feel unwell

• can be given rectally - good absorption but needs increased dose

what are adverse effects of metronidazole?

• mutagenic - problems with the foetus if used in pregnant animals

• neurotoxicity

• depression

• decreased appetite

what is the most likely bacteria causing this?

streptococci